Process for producing and method of cleaning polyarylene sulfide, and method of purifying organic solvent used for cleaning

ABSTRACT

A process for producing a poly(arylene sulfide), comprising the respective steps of a washing step of washing a polymer separated from a reaction mixture containing the polymer formed with an organic solvent, which is arranged after a polymerization step, a purification step of purifying the organic solvent recovered after the washing step to lower the content of an alkaline compound mixed therein, and a step of recycling the purified organic solvent through the washing step of the poly(arylene sulfide).

TECHNICAL FIELD

The present invention relates to a production process of a poly(arylenesulfide), and particularly to a production process of a poly(arylenesulfide), comprising the steps of washing a poly(arylene sulfide)obtained by a polymerization step in an organic amide solvent with anorganic solvent, recovering the organic solvent and then reducing thecontent of alkaline compounds such as methylamine contained in theorganic solvent recovered to recycle the purified organic solventthrough the washing step.

The present invention also relates to a washing method of a poly(arylenesulfide) obtained in a polymerization step. The present inventionfurther relates to a purification process of an organic solvent used inwashing of the poly(arylene sulfide).

BACKGROUND ART

Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) representedby poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) areengineering plastics excellent in heat resistance, chemical resistance,flame retardancy, mechanical properties, electrical properties,dimensional stability and the like. The PASs are commonly used in a widevariety of fields such as electrical and electronic equipments andautomotive equipments because they can be formed or molded into variouskinds of molded or formed products, films, sheets, fibers, etc. bygeneral melt processing techniques such as extrusion, injection moldingand compression molding.

As a typical production process of a PAS, is known a process, in whichan alkali metal sulfide that is a sulfur source is reacted with adihalo-aromatic compound in an organic amide solvent such asN-methyl-2-pyrrolidone. As the sulfur source, a combination of an alkalimetal hydrosulfide and an alkali metal hydroxide is also used.

Secondarily produced salts such as NaCl, oligomers, polymerization aids,decomposition products and/or the like are contained together with a PASformed in the reaction mixture by the polymerization reaction of thesulfur source with the dihalo-aromatic compound in the organic amidesolvent. It is difficult to sufficiently remove these impurities evenwhen the granular polymer formed is separated from the reaction mixtureby sieving and washed with water. In many cases, a step of washing thePAS formed with the same organic amide solvent as the polymerizationsolvent, or an organic solvent such as a ketone (for example, acetone)or an alcohol (for example, methanol) is thus arranged as a posttreatment step. Among these organic solvents (hereinafter also referredto as “washing solvents”) for washing, acetone and methanol are commonlyused because they are easily recovered by distillation owing to theirlow boiling points, and are excellent in detergency.

These washing solvents are repeatedly used in the washing step of theproduction of PASs. When a washing solvent is repeatedly used, a liquidcomponent such as a filtrate containing many impurities, an organicamide solvent, etc. in addition to the washing solvent is distilledafter the washing step to recover the washing solvent.

However, the present inventors have found that when a washing solventsuch as acetone is repeatedly used (i.e., “recycled through”) in thewashing step, the mere purification of the washing solvent bydistillation does not permit sufficiently removing a minute amount ofimpurities that adversely affect the quality of a PAS formed, and thephysical properties of the PAS after washing are adversely affected.

More specifically, it has been proven that when the washing solvent isrepeatedly recovered to reuse it in the washing step, the PAS after thewashing is colored, or the melt viscosity of the PAS is lowered in anextreme case. As a result, the washing solvent cannot be used furtherrepeatedly.

Further, although a PAS formed is treated with an aqueous solution ororganic solvent solution of an acid or a salt (for example, ammoniumchloride) of a weak alkali and a strong acid in a post treatment step inorder to raise the crystallization temperature (Tmc: also referred to as“melt crystallization temperature”) of the PAS, the effect by the acidor salt is reduced when the washing solvent is repeatedly recovered bydistillation to recycle it through the washing step, so that thecrystallization temperature is hard to be raised. When thecrystallization temperature of the PAS is lowered, the efficiency of amolding operation is reduced because, for example, the cycle ofinjection molding is elongated.

However, recognition as to purification of the washing solvent used inthe washing step of the PAS has heretofore been too low to proposeeffective means for solving such problems as described above. On theother hand, some methods have been proposed as to purification of theorganic amide solvent used in the polymerization step.

As a process for recovering N-methyl-2-pyrrolidone from a slurry of aPAS obtained by reacting an alkali metal sulfide with a dihalo-aromaticcompound in N-methyl-2-pyrrolidone to purify it, there has heretoforebeen proposed a process comprising adding a small amount of an alkalimetal hydroxide and/or an alkali metal carboxylate to a liquidcomprising N-methyl-2-pyrrolidone as a principal component, and thenconducting distillation (for example, Japanese Patent ApplicationLaid-Open No. 11-349566).

As a process for recovering N-methyl-2-pyrrolidone from a slurry of aPAS obtained by reacting an alkali metal sulfide with a dihalo-aromaticcompound in N-methyl-2-pyrrolidone to purify it, there has also beenproposed a process comprising adding a small amount of ammonia or amineto a liquid comprising N-methyl-2-pyrrolidone as a principal component,and then conducting distillation (for example, Japanese PatentApplication Laid-Open No. 11-354769).

However, the processes for recovering and purifying the organic amidesolvent by adding such alkaline compound(s) are not suitable forrecovering and purifying the washing solvents such as acetone andmethanol. In fact, even when any of the above-described processes isapplied to a liquid comprising such a washing solvent as a principalcomponent after the washing step, lowering of the crystallizationtemperature of a PAS formed or coloring thereof cannot be prevented whenthe washing solvent recovered is reused in the washing step.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a production processof a poly(arylene sulfide), comprising the steps of washing apoly(arylene sulfide) obtained by a polymerization step in an organicamide solvent with an organic solvent, recovering the organic solventand then reducing the content of impurities, which are contained in theorganic solvent recovered and adversely affect the quality of the PASformed, to recycle the purified organic solvent through the washingstep.

Another object of the present invention is to provide a novel washingmethod of a poly(arylene sulfide), in which an organic solvent used inwashing is recycled through a washing step.

A further object of the present invention is to provide an effectivepurification process of an organic solvent used in washing.

The present inventors have carried out an extensive investigation with aview toward achieving the above objects. As a result, the inventors havepaid attention to the fact that in a production process of a PAS byheating and polymerizing a dihalo-aromatic compound and a sulfur sourcesuch as an alkali metal sulfide in an organic amide solvent, an alkalinecompound such as methylamine is secondarily produced, which isconsidered to be attributable to decomposition of the organic amidesolvent. It has been found that when this alkaline compound is mixed ina washing solvent, the compound is difficult to be removed even bydistillation and accumulates in the washing solvent during repeated useand recovery of the washing solvent.

Thus, the present inventors have carried out a further investigation. Asa result, it has been found that when an inorganic solvent such ashydrochloric acid is added to the washing solvent recovered from thewashing step to convert the alkaline compound such as methylamine to asalt with the inorganic acid, and distillation is then conducted, thecontent of the alkaline compound, which greatly adversely affects thephysical properties of the PAS, can be effectively lowered.

It has also been found that the alkaline compound such as methylaminecan be markedly lowered even by treating the washing solvent recoveredfrom the washing step with activated carbon.

As a result, it has been proven that even when the washing solvent suchas acetone is recycled repeatedly at least several tens times, theadverse influence on the physical properties of the PAS can be markedlyrelaxed. The present invention has been led to completion on the basisof these findings.

According to the present invention, there is provided a process forproducing a poly(arylene sulfide), which comprises the respective stepsof:

-   (1) a polymerization step of heating and polymerizing at least one    sulfur source selected from the group consisting of alkali metal    sulfides and alkali metal hydrosulfides and a dihalo-aromatic    compound in an organic amide solvent (A) after an alkali metal    hydroxide is added as needed,-   (2) a separation step of separating a polymer formed from the    reaction mixture containing the polymer after the polymerization    step,-   (3) a washing step of washing the polymer separated with an organic    solvent (B),-   (4) a purification step of purifying the organic solvent (B)    recovered after the washing step to lower the content of an alkaline    compound mixed therein, and-   (5) a step of recycling the organic solvent (B) purified through the    washing step of the poly(arylene sulfide).

According to the present invention, there is also provided a process forproducing a poly(arylene sulfide), comprising the step of washing thepoly(arylene sulfide) obtained by a polymerization step in an organicamide solvent (A) with an organic solvent (B), wherein the washing isconducted with an organic solvent (B), the content of an alkalinecompound in which has been lowered to at most 3,000 ppm by weight, toprovide a poly(arylene sulfide) having a yellow index (YI) of at most15.0. In this production process, when a treatment for raising acrystallization temperature is conducted after the washing, apoly(arylene sulfide) having a crystallization temperature (Tmc) of atleast 200° C. and a yellow index (YI) of at most 11.0 is provided.

According to the present invention, there is further provided a methodfor washing a poly(arylene sulfide) obtained by a polymerization step inan organic amide solvent (A) with an organic solvent (B), comprisingrecovering the organic solvent (B) used in a washing step to recycle itthrough the washing step, wherein the content of an alkaline compound inthe organic solvent (B) recovered at that time is lowered to at most3,000 ppm by weight to recycle the organic solvent (B) thus treatedthrough the washing step.

According to the present invention, there is still further provided aprocess for purifying an organic solvent used in washing, whichcomprises washing a poly(arylene sulfide) obtained by a polymerizationstep in an organic amide solvent (A) with an organic solvent (B),recovering the organic solvent (B) and then adding an inorganic acid tothe organic solvent (B) recovered to conduct distillation.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Sulfur Source:

In the present invention, at least one sulfur source selected from thegroup consisting of alkali metal hydrosulfides and alkali metal sulfidesis used as a sulfur source. As examples of the alkali metal sulfides,may be mentioned lithium sulfide, sodium sulfide, potassium sulfide,rubidium sulfide, cesium sulfide and mixtures of two or more compoundsthereof. These alkali metal sulfides are generallycommercially-available and used in the form of a hydrate. Examples ofthe hydrate include sodium sulfide nonahydrate (Na₂S.9H₂O) and sodiumsulfide pentahydrate (Na₂S.5H₂O). The alkali metal sulfide may be usedas an aqueous mixture.

As a sulfur source, an alkali metal hydrosulfide may be used incombination with an alkali metal hydroxide. As examples of the alkalimetal hydrosulfide, may be mentioned lithium hydrosulfide, sodiumhydrosulfide, potassium hydrosulfide, rubidium hydrosulfide, cesiumhydrosulfide and mixtures of two or more compounds thereof. The alkalimetal hydrosulfide may be used in any form of an anhydride, a hydrateand an aqueous solution. Among these, sodium hydrosulfide and lithiumhydrosulfide are preferred in that they are industrially available onthe cheap. The alkali metal hydrosulfide is preferably used as anaqueous mixture (i.e., a mixture with water having fluidity) such as anaqueous solution from the viewpoints of treatment operation, metering,etc.

In general, a small amount of an alkali metal sulfide is secondarilyproduced in a production process of the alkali metal hydrosulfide. Asmall amount of the alkali metal sulfide may be contained in the alkalimetal hydrosulfide used in the present invention. In this case, thetotal molar quantity of the alkali metal hydrosulfide and alkali metalsulfide becomes a sulfur source charged after a dehydration step.

Examples of the alkali metal hydroxide include lithium hydroxide, sodiumhydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide andmixtures of two or more compounds thereof. Among these, sodium hydroxideand lithium hydroxide are preferred in that they are industriallyavailable on the cheap. The alkali metal hydroxide is preferably used asan aqueous mixture such as an aqueous solution.

In the production process according to the present invention, examplesof water to be dehydrated in a dehydration step includes water ofhydration, a water medium of an aqueous solution and water secondarilyproduced by a reaction of the alkali metal hydrosulfide with the alkalimetal hydroxide, or the like.

2. Dihalo-Aromatic Compound:

The dihalo-aromatic compound used in the present invention is adihalogenated aromatic compound having 2 halogen atoms directly bondedto the aromatic ring. Specific examples of the dihalo-aromatic compoundinclude o-dihalobenzene, m-dihalobenzene, p-dihalobenzene,dihalotoluene, dihalonaphthalene, methoxy-dihalobenzene, dihalobiphenyl,dihalobenzoic acid, dihalodiphenyl ether, dihalodiphenyl sulfone,dihalodiphenyl sulfoxide and dihalodiphenyl ketone.

Here, the halogen atom means each atom of fluorine, chlorine, bromineand iodine atoms, and 2 halogen atoms in the dihalo-aromatic compoundmay be the same or different from each other. These dihalo-aromaticcompounds may be used either singly or in any combination thereof.

The amount of the dihalo-aromatic compound charged is generally 0.90 to1.50 mol, preferably 0.95 to 1.20 mol, more preferably 1.00 to 1.09 molper mol of the sulfur source (alkali metal sulfide and/or alkali metalhydrosulfide) remaining in the system after the dehydration step.

3. Molecular Weight Modifier, Branching or Crosslinking Agent:

In order to, for example, form a terminal of a specific structure in aPAS formed or regulate a polymerization reaction or a molecular weight,a monohalo-compound (may not be always an aromatic compound) may be usedin combination. In order to form a branched or crosslinked polymer, apolyhalo-compound (may not be always an aromatic compound), to which atleast 3 halogen atoms are bonded, an active hydrogen-containinghalogenated aromatic compound, a halogenated aromatic nitro compound orthe like may also be used in combination. As the polyhalo-compound as abranching or crosslinking agent is preferred trihalobenzene.

4. Organic Amide Solvent:

In the present invention, an organic amide solvent that is an aproticpolar organic solvent is used as a solvent for a dehydration reactionand a polymerization reaction. The organic amide solvent is preferablystable to an alkali at a high temperature.

Specific examples of the organic amide solvent include amide compoundssuch as N,N-dimethylformamide and N,N-dimethylacetamide;N-alkylcaprolactam compounds such as N-methyl-ε-caprolactam;N-alkylpyrrolidone compounds or N-cycloalkylpyrrolidone compound such asN-methyl-2-pyrrolidone and N-cyclohexyl-2-pyrrolidone;N,N-dialkyl-imidazolidinone compounds such as1,3-dialkyl-2-imidazolidinones; tetraalkylurea compounds such astetramethylurea; and hexaalkylphosphoric triamide compounds such ashexamethylphosphoric triamide. These organic amide solvents may be usedeither singly or in any combination thereof.

Among these organic amide solvents, N-alkyl-pyrrolidone compounds,N-cycloalkylpyrrolidone compounds, N-alkylcaprolactam compounds andN,N-dialkyl-imidazolidinone compounds are preferred, andN-methyl-2-pyrrolidone, N-methyl-ε-caprolactam and1,3-dialkyl-2-imidazolidinones are particularly preferably used.

The amount of the organic amide solvent used in the polymerizationreaction in the present invention is generally within a range of 0.1 to10 kg per mol of the sulfur source.

5. Polymerization Aid:

In order to promote the polymerization reaction to obtain a PAS having ahigh polymerization degree in a short period of time, various kinds ofpolymerization aids may be used in the present invention as needed.Specific examples of the polymerization aids include metal salts oforganic sulfonic acids, lithium halides, metal salts of organiccarboxylic acids and alkali metal salts of phosphoric acid, which aregenerally publicly known as polymerization aids for PASs. Among these,metal salts of organic carboxylic acids are particularly preferredbecause they are cheap.

The amount of the polymerization aid used varies according to the kindof the compound used. However, it is generally within a range of 0.01 to10 mol per mol of the sulfur source charged.

6. Dehydration Step:

A dehydration step is preferably arranged as a preliminary step for apolymerization step in order to control the amount of water in thereaction system. The dehydration step is performed by a processcomprising heating and reacting an alkali metal sulfide and/or an alkalimetal hydrosulfide in an organic amide solvent in the presence of analkali metal hydroxide as needed, desirably, under an inert gasatmosphere and discharging water outside the system by distillation.

Since the alkali metal sulfide is generally used as a hydrate or aqueousmixture, it contains more water than the polymerization reaction needs.When the alkali metal hydrosulfide is used as a sulfur source, an alkalimetal hydroxide of the order of an equimolar amount is added to allowthe alkali metal hydrosulfide to react therewith in situ in an organicamide solvent.

In the dehydration step, the dehydration is conducted until the contentof water comprising water of hydration (water of crystallization), awater medium, secondarily produced water, etc. is lowered within a rangeof necessary amounts. In the dehydration step, the dehydration isconducted until the water content in the polymerization reaction systemis reduced to generally about 0.3 to 5 mol, preferably about 0.5 to 2mol per mol of the sulfur source. When the water content has become toolow in the dehydration step, water may be added prior to apolymerization step to adjust the water content to a desired value.

The charging of these raw materials into a reaction vessel is conductedwithin a temperature range of generally from ordinary temperature to300° C., preferably from ordinary temperature to 200° C. The charging ofthe raw materials may not be in order, and the respective raw materialsmay be additionally charged in the course of the dehydration process. Anorganic amide solvent is used as a solvent used in the dehydration step.This solvent is preferably the same as the organic amide solvent used inthe polymerization step, and N-methyl-2-pyrrolidone is particularlypreferred. The amount of the organic amide solvent used is generallyabout 0.1 to 10 kg per mol of the sulfur source charged in the reactionvessel.

The dehydration process is conducted by heating the mixture aftercharging the raw materials into the reaction vessel in a temperaturerange of generally up to 300° C., preferably from 100 to 250° C. forgenerally 15 minutes to 24 hours, preferably 30 minutes to 10 hours.Heating methods include a method of retaining a fixed temperature, amethod of raising the temperature either stepwise or continuously and amethod of combining both methods. The dehydration step is conducted by,for example, a batch system, a continuous system or a combined systemthereof.

An apparatus for conducting the dehydration step may be the same as areaction vessel (reactor) used in the subsequent polymerization step ordifferent from it. A material of the apparatus is preferably a corrosionresistant material such as titanium. In the dehydration step, a part ofthe organic amide solvent is generally discharged together with wateroutside the reaction vessel. At that time, hydrogen sulfide isdischarged as a gas outside the system.

7. Polymerization Step:

The polymerization step is conducted by charging a dihalo-aromaticcompound into the mixture after completion of the dehydration step andheating the sulfur source and the dihalo-aromatic compound in an organicamide solvent. When a polymerization vessel different from the reactionvessel used in the dehydration step is used, the mixture after thedehydration step and the dihalo-aromatic compound are charged into thepolymerization vessel. After the dehydration step and before thepolymerization step, the amounts of the organic amide solvent andcoexisting water may be controlled as needed. Before the polymerizationstep or during the polymerization step, a polymerization aid and otheradditives may be mixed.

The mixing of the mixture obtained after completion of the dehydrationstep with the dihalo-aromatic compound is conducted within a temperaturerange of generally from 100 to 350° C., preferably from 120 to 330° C.When the respective components are charged into the polymerizationvessel, no particular limitation is imposed on the order of charging,and both components are charged in small portions or at a time.

The polymerization reaction is conducted at generally 100 to 350° C.,preferably 120 to 330° C., more preferably 170 to 290° C. As a heatingmethod for this reaction, is used a method of retaining a fixedtemperature, a method of raising the temperature either stepwise orcontinuously or a combination of both methods. The polymerizationreaction time is within a range of generally from 10 minutes to 72hours, desirably from 30 minutes to 48 hours. The amount of the organicamide solvent used in the polymerization step is within a range ofgenerally 0.1 to 10 kg, preferably 0.15 to 1 kg per mol of the chargedsulfur source existing in the polymerization step. The amount may bechanged in the course of the polymerization reaction so far as it fallswithin this range.

The content of water upon the beginning of the polymerization reactionis preferably controlled within a range of generally 0.3 to 5 mol permol of the sulfur source charged. When it is intended to obtain alow-molecular weight polymer or oligomer, or a special polymerizationprocess is adopted, however, the water content may be changed outsidethis range. For example, the water content may be controlled within arange of 0.1 to 15 mol, preferably 0.5 to 10 mol per mol of the sulfursource such as an alkali metal sulfide. The water content may beincreased in the course of the polymerization reaction or decreased bydistillation to the contrary.

Examples of polymerization processes comprising increasing the watercontent in the course of the polymerization reaction include a processcomprising conducting a reaction at a temperature of 170 to 270° C.,preferably 180 to 235° C. in the presence of water in an amount of 0.5to 2.4 mol, preferably 0.5 to 2.0 mol per mol of the sulfur sourcecharged to control a conversion of the dihalo-aromatic compound to 50 to98 mol %, adding water so as to bring about a state that water exists ina proportion of more than 2.0 mol, but up to 10 mol, preferably 2.5 to7.0 mol per mol of the sulfur source charged, and then heating thereaction system to a temperature of 245 to 290° C., thereby continuingthe reaction. When the dehydration step is arranged in this productionprocess, the charged sulfur source means an amount of the sulfur sourceremaining in the reaction vessel after the dehydration step.

As a particularly preferable production process, may be mentioned aprocess comprising conducting a polymerization reaction in thepolymerization step by an at least two-stage polymerization processcomprising:

-   (1) Step 1 of heating a reaction mixture containing the organic    amide solvent, the sulfur source (A) and the dihalo-aromatic    compound (B) to 170 to 270° C. in the presence of water in an amount    of 0.5 to 2.0 mol per mol of the sulfur source (A) charged to    conduct a polymerization reaction, thereby forming a prepolymer that    a conversion of the dihalo-aromatic compound is 50 to 98%, and-   (2) Step 2 of controlling the amount of water in the reaction system    so as to bring about a state that water exists in a proportion of    more than 2.0 mol, but up to 10 mol per mol of the sulfur source (A)    charged, and heating the reaction system to 245 to 290° C., thereby    continuing the polymerization reaction.

In Step 1, it is desirable to form a prepolymer having a melt viscosityof 0.5 to 30 Pa·s as measured at a temperature of 310° C. and a shearrate of 1,216 sec⁻¹. In Step 2, the polymerization reaction is continueduntil the melt viscosity of the prepolymer is increased.

Water may be added at a final stage of the polymerization reaction orupon completion thereof to increase the water content for the purpose oflowering the contents of common salt secondarily produced and impuritiesin the polymer formed or collecting the polymer in the form ofparticles. To the polymerization step according to the presentinvention, may be applied many of other publicly known polymerizationprocesses or modified processes thereof, and the present invention isparticularly not limited to a particular polymerization process.

The polymerization reaction system may be a batch system, a continuoussystem or a combination of both systems. In the batch-wisepolymerization, 2 or more reaction vessels may be used for the purposeof shortening the polymerization cycle time.

8. Post Treatment Step:

In the production process according to the present invention, a posttreatment after the polymerization reaction may be conducted inaccordance with a method known per se in the art. For example, aftercompletion of the polymerization reaction, a product slurry cooled isseparated by filtration as it is or after diluted with water or thelike, and the resulting filter cake is washed and filtered repeatedly,and then dried, whereby a PAS can be collected. The product slurry maybe subjected to sieving as it is in a high-temperature state, therebycollecting the polymer.

After the separation (sieving) by filtration, the PAS is washed with thesame organic amide solvent as the polymerization solvent, or an organicsolvent such as a ketone (for example, acetone) or an alcohol (forexample, methanol). The PAS may be washed with hot water or the like.The PAS formed may also be treated with an acid or a salt such asammonium chloride.

9. Recovery and Purification of Washing Solvent:

In the present invention, after the separation step of separating thepolymer from the reaction mixture containing the polymer formed afterthe polymerization step, a washing step of washing the polymer separatedwith an organic solvent (B) is arranged, and a purification step ofpurifying the organic solvent (B) recovered to lower the content of analkaline compound mixed therein is further arranged after the washingstep. The purified organic solvent (B) may be recycled through thewashing step of the PAS. More specifically, the organic solvent (B) usedin the washing step of the PAS may be purified to reuse it in a washingstep of another PAS, and the number of times of reuse may also beincreased.

In the washing step, the polymer is generally washed by bringing thepolymer into contact with the organic solvent (B) . More specifically, awet cake of the polymer separated in the separation step and the organicsolvent (B) are mixed and stirred. The amount of the organic solvent (B)used in the washing is of the order of generally 1 to 10 times,preferably 2 to 8 times as much as the weight of the polymer. After thewashing step, the polymer is separated from a liquid component (C)containing the organic solvent (B) used in the washing. After thepolymerization step, the PAS purified is generally collected as agranular polymer, and so the polymer can be sifted by means of a screen.Even in the separation step, thus, the reaction mixture containing thepolymer formed can be subjected to sieving through a screen to separatethe polymer.

In the separation step, it is preferred that a solid-liquid mixedcomponent passed through a screen and containing the organic amidesolvent (A) be separated into a solid component and a liquid component(D1), and an organic solvent (B) be added to the solid componentseparated to separate the mixture into a liquid component (D2)containing the organic amide solvent (A) and organic solvent (B) and asolid component in that the organic amide solvent (A) is efficientlyrecovered. Since the solid component is composed of fine particles ofthe by-product salt and/or the like, it is desirable to use acentrifugal separator or decanter in the separation between the liquidcomponents (D1 and D2) and the solid components.

After the washing step, the organic solvent (B) is recovered from theliquid component (C) or a mixture of the liquid component (C) and theliquid component (D1 and/or D2) by distillation. The distillation may begenerally simple distillation. However, the organic solvent (B)recovered by the mere simple distillation contains an alkaline compoundsuch as methylamine, and the alkaline compound accumulates in theorganic solvent (B) for washing when the washing and recovery areconducted repeatedly.

In the present invention, thus, an inorganic acid is added to theorganic solvent (B) recovered by the distillation in the purificationstep to conduct distillation again, thereby lowering the content of thealkaline compound. Examples of the inorganic acid include hydrochloricacid, sulfuric acid and nitric acid. Among these, hydrochloric acid ispreferred. When the inorganic acid is added to the organic solvent (B)recovered by the distillation, the inorganic acid reacts with thealkaline compound such as methylamine to form a salt (for example,hydrochloride), and the pH of the organic solvent is lowered. It ispreferred that the inorganic acid be added to the organic solvent (B)recovered to adjust the pH thereof to lower than 10.0 in that thealkaline compound such as methylamine is efficiently removed.

In the purification step, the content of the alkaline compound can belowered by bringing the organic solvent (B) recovered into contact withactivated carbon. In the process according to the present invention, itis preferred to use the inorganic acid such as hydrochloric acid fromthe viewpoints of cost, pH control and the like.

The organic solvent (B) used in the washing step is preferably a ketoneor alcohol. Acetone and methanol are more preferred, and acetone isparticularly preferred. The alkaline compound is typically methylamine.According to the process of the present invention, the content of thealkaline compound (for example, methylamine) in the organic solvent (B)recovered by the distillation is lowered to at most 3,000 ppm,preferably at most 2,000 ppm, more preferably at most 1,000 ppm.Incidentally, ppm is based on the weight.

10. PAS:

According to the production process of the present invention, there canbe provided a PAS having a crystallization temperature (Tmc) of at least200° C., preferably at least 210° C., more preferably at least 220° C. ayellow index (YI) of at most 11.0, preferably at most 10.0, morepreferably at most 7.0 when a treatment for raising a crystallizationtemperature. is conducted. Even when no treatment for raising acrystallization temperature is conducted, a PAS having a yellow index(YI) of at most 15.0, preferably at most 13.0, more preferably at most11.0 can be provided.

No particular limitation is imposed on the melt viscosity (as measuredat a temperature of 310° C. and a shear rate of 1,216 sec⁻¹) of the PASaccording to the present invention. However, it is preferably within arange of 30 to 800 Pa·s, more preferably 40 to 500 Pa·s. When thepolymerization reaction is conducted by 2 stages, a PAS having a meltviscosity exceeding the melt viscosity of a prepolymer formed in thefirst-stage step (Step 1) is obtained in the second-stage step (Step 2).

The PASs obtained by the production process according to the presentinvention may be molded or formed into various injection-molded productsor extruded products such as sheets, films, fibers and pipes eithersingly or by incorporating various kinds of inorganic fillers, fibrousfillers and/or various kinds of synthetic resins, if desired, as it isor after oxidized and crosslinked.

Since the PASs obtained by the process according to the presentinvention are little in lot-to-lot variation of melt viscosity, theprocessing thereof can be stably conducted, and the resulting formed ormolded products are provided as high-quality products little invariations of various properties. The PAS is particularly preferablypoly(phenylene sulfide) (PPS).

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples. Analyzing methods andmeasuring methods are as follows.

(1) Determination of Methylamine:

An 0.1 N aqueous solution of sodium hydroxide of the same weight asacetone was added to acetone [acetone/aqueous solution of sodiumhydroxide=1/1 (weight ratio)] to analyze the content of methylaminecontained in acetone by gas chromatography.

<Conditions of Gas Chromatographic Analysis>

-   -   Apparatus: Hitachi G-3000,    -   Temperature of vaporizing chamber: 180° C.,    -   Column: packing material, PEG-20M+KOH (20%+5%), carrier, Uniport        B60/80 mesh, 3 mm in diameter×3 m,    -   Column temperature: 100° C.,    -   Detector: FID (flame thermionic detector), 150° C.,    -   Carrier gas: nitrogen, 30 ml/min, and    -   Amount of sample: 2 μl.

Incidentally, a value was found from a calibration curve usingcommercially available methylamine.

(2) Measurement of Crystallization Temperature (Tmc):

After a polymer was heated and melted by means of a hot press controlledto 320° C., the polymer was quenched to prepare an amorphous sheet.About 10 mg of an amorphous sheet portion was taken as a sample formeasurement out of the amorphous sheet to measure its crystallizationtemperature (Tmc) under temperature-lowering conditions by means of adifferential scanning calorimeter (DSC). Described specifically, thecrystallization temperature was measured under conditions that thesample was heated up to 340° C. in a nitrogen gas atmosphere (20 ml/min)and held at that temperature for 10 minutes and then cooled at a rate of10° C./min.

(3) Measurement of Color Tone:

A polymer was pressed under 15 MPa at room temperature for 1 minute bymeans of an electrical press machine to prepare tablets. The tabletswere used as a sample for measurement to measure a color tone by meansof TC-1800 (manufactured by Tokyo Denshoku Gijutsu Center) in accordancewith a reflected light measuring method under conditions of standardlight C, a visual field of 2° and a calorimetric system. Prior to themeasurement, the apparatus was calibrated by a standard white plate. Themeasurement was conducted at 3 points as to each sample, and an averagevalue thereof was calculated out. The color tone of the sample wasindicated by a yellow index (YI).

(4) Melt Viscosity:

A melt viscosity was measured by means of Capirograph 1-C (manufacturedby Toyo Seiki Seisakusho, Ltd.) using about 20 g of a dry polymer. Atthis time, a flat die of 1 mm in diameter×10 mm in length was used as acapillary, and the temperature was set to 310° C. After the polymersample was placed in the apparatus and held for 5 minutes, the meltviscosity was measured at a shear rate of 1,216 sec⁻¹.

Referential Example 1

(1) Polymerization Step:

After 1,300 kg of N-methyl-2-pyrrolidone (hereinafter abbreviated as“NMP”) was charged into a reaction vessel and heated to 150° C., 300 kg(3.42 kmol in terms of NaSH) of sodium hydrosulfide having aconcentration of 64% by weight and 185 kg (3.47 kmol) of sodiumhydroxide having a concentration of 75% by weight were charged, andheating was conducted until the temperature within the reaction vesselreached 200° C. to conduct a dehydration reaction. The amount ofhydrogen sulfide volatilized out by this dehydration step was 2 kg (0.06kmol). This value was used to calculate out an amount of the sulfursource within the reaction vessel, and it was found to be 3.36 kmol.

This reaction vessel was charged with 500 kg (3.40 kmol) ofp-dichlorobenzene (hereinafter abbreviated as “pDCB”) [pDCB/sulfursource (molar ratio)=1.012], and the contents were heated up to 220° C.to conduct a reaction for 5 hours. The reaction vessel was then chargedwith 100 kg of water, and the contents were heated up to 260° C. toconduct a reaction for 5 hours. After completion of the polymerizationreaction, the reaction vessel was cooled near to room temperature toobtain a reaction mixture as a slurry.

(2) Separation Step of Polymer:

The reaction mixture was sifted through a screen having a sieve openingof 150 μm (100 mesh) to separate it into a wet cake containing agranular polymer on the screen and a component passed through thescreen.

(3) Washing Step of Polymer:

After the wet cake was brought into contact with high-purity acetone(containing no methylamine; the same shall apply hereinafter) in anamount 5 times as much as the weight of the polymer while stirring atroom temperature for 10 minutes, it was sifted through a screen having asieve opening of 150 μm to separate it into a polymer component capturedon the screen and a component passed through the screen. Theabove-described process was conducted as to the polymer componentcaptured on the screen once again. The whole amount of a liquidcomponent (C) passed through the screen was recovered.

The polymer component captured on the screen was brought into contactwith ion-exchanged water in an amount 5 times as much as the weight ofthe polymer while stirring at room temperature for 10 minutes, and itwas then sifted through a screen having a sieve opening of 150 μm tocollect a polymer component captured on the screen again. This processwas further repeated twice. Thereafter, the polymer component capturedon the screen and collected was brought into contact with a 0.6% byweight aqueous solution of acetic acid in an amount 5 times as much asthe weight of the polymer for 40 minutes and sifted through a screenhaving a sieve opening of 150 μm to collect a polymer component capturedon the screen.

Thereafter, the polymer component captured on the screen was broughtinto contact with ion-exchanged water in an amount 5 times as much asthe weight of the polymer while stirring at room temperature for 10minutes, it was then sifted through a screen having a sieve opening of150 μm, and a polymer component captured on the screen was collected anddried at 105° C.

The polymer collected by the above-described processes was granular andhad a melt viscosity of 143 Pa·s, a Tmc of 235° C. and a YI of 5.6.

(4) Recovery Step of Polymerization Solvent:

The component passed through the screen by the first sieving of thereaction mixture containing the reaction product in the above-describedseparation step of polymer contains NMP that is a polymerizationsolvent, secondarily produced salts, a low-molecular weight PPScomponent, water and organic impurities such as methylamine.

In order to recover NMP from these components, the component passedthrough the screen was separated into a liquid component (D1) and asolid component by centrifugal separation or decantation (decanter), thesolid component was brought into contact with high-purity acetone in anamount 5 times as much as the weight of the polymer, and the resultantmixture was separated into a liquid component and a solid component torecover the liquid component (D2).

(5) Recovery Step of Washing Solvent:

The liquid components (C), (D1) and (D2) recovered in the respectiveprocesses described above were mixed, and the resultant mixture wassubjected to simple distillation to recover acetone. The recoveredacetone contained methylamine at a concentration of 200 ppm. The pH of amixture (acetone/water=1/1 weight ratio) obtained by taking out a partof the acetone and adding the same weight of water thereto was 11.8.

(6) Repetitive Experiments of Respective Steps:

The steps (1) to (5) were performed in the same manner as describedabove except that the acetone (methylamine content: 200 ppm) recoveredin the recovery step (5) of washing solvent was used in the washing stepof polymer and the recovery step of polymerization solvent. In therecovery step (5) of washing solvent, acetone was recovered by simpledistillation. The content of methylamine in the acetone (acetone by thesecond recovery) recovered increased to 420 ppm.

The above-described process was conducted repeatedly to recover acetoneby simple distillation in the recovery step (5) of washing solvent. Thecontent of methylamine in the acetone recovered was 1,230 ppm in thefifth recovery, 2,380 ppm in the tenth recovery or 4,710 ppm in thetwentieth recovery. The results are shown in Table 1. TABLE 1Purification of acetone by simple distillation (pH before distillation =11.8) Ref. Ordinal No. of times 1st 2nd 5th 10th 20th Ex. 1 of recoveryof acetone Content of methylamine 200 420 1230 2380 4710 (ppm)

As apparent from the results shown in Table 1, it is understood that thepurification of acetone is insufficient according to the recoveringmethod of acetone by the simple distillation, and the content ofmethylamine markedly increases when the same acetone is used repeatedlyin the washing step.

Example 1

The steps (1) to (5) were performed in the same manner as in ReferentialExample 1. As a result, acetone containing 200 ppm of methylamine wasrecovered. After aqueous hydrochloric acid was added to the acetonerecovered to adjust it so as to give a water content of about 30% byweight and a pH of 6.0, the acetone was purified by conducting simpledistillation again.

The content of methylamine in the acetone (acetone by the first recoveryby the simple redistillation) purified and recovered in such a mannerlowered to 20 ppm. The above-described whole process was conductedrepeatedly. As a result, the content of methylamine in the acetonerecovered was 45 ppm in the fifth recovery or 80 ppm in the tenthrecovery. The results are shown in Table 2.

Example 2

The steps (1) to (5) were performed in the same manner as in ReferentialExample 1. As a result, acetone containing 200 ppm of methylamine wasrecovered. After aqueous hydrochloric acid was added to the acetonerecovered to adjust it so as to give a water content of about 30% byweight and a pH of 9.0, the acetone was purified by conducting simpledistillation again.

The content of methylamine in the acetone purified and recovered in sucha manner lowered to 35 ppm. The above-described whole process wasconducted repeatedly. As a result, the content of methylamine in theacetone recovered was 80 ppm in the fifth recovery or 170 ppm in thetenth recovery.

Example 3

After aqueous hydrochloric acid was added to the acetone (methylaminecontent: 4,710 ppm) recovered in the twentieth recovery in ReferentialExample 1 to adjust it so as to give a water content of about 30% byweight and a pH of 5.0, an acetone component was recovered by simpledistillation. As a result, the content of methylamine therein lowered to190 ppm. The result is shown in Table 2.

Example 4

The steps (1) to (5) were performed in the same manner as in ReferentialExample 1. As a result, acetone containing 200 ppm of methylamine wasrecovered. After aqueous hydrochloric acid was added to the acetonerecovered to adjust it so as to give a water content of about 30% byweight and a pH of 10.0, the acetone was purified by conducting simpledistillation again.

The content of methylamine in the acetone purified and recovered in sucha manner lowered to 140 ppm. The above-described whole process wasconducted repeatedly. As a result, the content of methylamine in theacetone recovered was 520 ppm in the fifth recovery or 1,280 ppm in thetenth recovery. The results are shown in Table 2. TABLE 2 Methylaminecontent in acetone purified by Acetone before simple redistillationsimple redistillation Methylamine content (ppm) (Ordinal No. in acetoneof times of recovery by Inorganic recovered by simple simpleredistillation) Ex. acid pH distillation (ppm) First Fifth Tenth 1Aqueous 6.0 200 20 45 80 HCl (first recovery) 2 Aqueous 9.0 200 35 80170 HCl (first recovery) 3 Aqueous 5.0 4710 190 — — HCl (20th recovery)4 Aqueous 10.0 200 140 520 1280 HCl (first recovery)

As apparent from the results shown in Table 2, it is understood thatwhen aqueous hydrochloric acid is added to acetone recovered by simpledistillation, and simple redistillation is then conducted, the contentof methylamine contained in acetone can be markedly lowered. Example 3indicates that even in the acetone having a methylamine content of 4,710ppm recovered in the twentieth recovery in Referential Example 1, thecontent of methylamine contained in the acetone can be markedly loweredwhen aqueous hydrochloric acid is added to the acetone, and simpleredistillation is then conducted. When the relevance between pH and theeffect of lowering the methylamine content is observed, it is understoodthat the methylamine content can be more effectively lowered byadjusting the pH of acetone to lower than 10.0.

Example 5

Activated carbon was brought into contact with the acetone (methylaminecontent: 4,710 ppm) recovered in the twentieth recovery in ReferentialExample 1. As a result, the methylamine content lowered to 50 ppm.

Example 6

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 80 ppm recoveredin the tenth recovery in Example 1 was used in place of the high-purityacetone in the washing step of polymer in Referential Example 1. As aresult, the resultant polymer had a melt viscosity of 140 Pa·s, a Tmc of232° C. and a YI of 5.2. The results are shown in Table 3.

Example 7

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 190 ppmrecovered in the first recovery in Example 3 was used in place of thehigh-purity acetone in the washing step of polymer in ReferentialExample 1. The results are shown in Table 3.

Example 8

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 520 ppmrecovered in the fifth recovery in Example 4 was used in place of thehigh-purity acetone in the washing step of polymer in ReferentialExample 1. The results are shown in Table 3.

Example 9

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 1,280 ppmrecovered in the tenth recovery in Example 4 was used in place of thehigh-purity acetone in the washing step of polymer in ReferentialExample 1. The results are shown in Table 3.

Example 10

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 2,380 ppmrecovered in the tenth recovery in Referential Example 1 was used inplace of the high-purity acetone in the washing step of polymer inReferential Example 1. The results are shown in Table 3.

Comparative Example 1

The process was performed in the same manner as in Referential Example 1except that the acetone having a methylamine content of 4,710 ppmrecovered in the twentieth recovery in Referential Example 1 was used inplace of the high-purity acetone in the washing step (first washing) ofpolymer in Referential Example 1. The results are shown in Table 3.TABLE 3 Ref. Comp. Ex. 1 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 1Methylamine 0 80 190 520 1280 2380 4710 content in acetone (ppm) Melt143 140 140 140 139 138 137 viscosity (Pa · s) Tmc (° C.) 235 232 230228 211 202 190 YI 5.6 5.2 6.0 6.5 7.3 10.4 12.6

As apparent from the results shown in Table 3, a PPS having acrystallization temperature (Tmc) of at least 200° C. and a YI of atmost 11.0 can be obtained by purifying acetone so as to give amethylamine content of at most 3,000 ppm when acetone used in washing isused in another washing step. From the viewpoints of Tmc and YI, thecontent of methylamine in acetone recycled is preferably at most 2,000ppm, more preferably at most 1,000 ppm, particularly preferably at most500 ppm.

Referential Example 2

The process was performed in the same manner as in Referential Example 1except that the treatment with the aqueous solution of acetic acid wasnot conducted. The polymer collected by this process had a meltviscosity of 250 Pa·s, a Tmc of 185° C. and a YI of 8.5. The results areshown in Table 4.

Example 11

The process was performed in the same manner as in Referential Example 2except that the acetone having a methylamine content of 1,280 ppmrecovered in the tenth recovery in Example 4 was used in place of thehigh-purity acetone in the washing step of polymer in ReferentialExample 2. The results are shown in Table 4.

Comparative Example 2

The process was performed in the same manner as in Referential Example 2except that the acetone having a methylamine content of 4,710 ppmrecovered in the twentieth recovery in Referential Example 1 was used inplace of the high-purity acetone in the washing step of polymer inReferential Example 2. The results are shown in Table 4. TABLE 4 Ref.Example 2 Example 11 Comp. Example 2 Methylamine 0 1280 4710 content inacetone (ppm) Melt viscosity 250 250 252 (Pa · s) Tmc (° C.) 185 182 180YI 8.5 8.6 15.8

INDUSTRIAL APPLICABILITY

According to the present invention, there can be provided productionprocesses of PASs, comprising the steps of washing a PAS obtained by apolymerization step in an organic amide solvent with an organic solvent,recovering the organic solvent and then reducing the content ofimpurities, which are contained in the organic solvent recovered andadversely affect the quality of the PAS formed, to recycle the purifiedorganic solvent through the washing step.

According to the production processes of the present invention, the PASobtained is prevented from greatly deteriorating its quality such ascrystallization temperature (Tmc) and yellow index (YI) even when theorganic solvent for washing is recovered and used repeatedly.

According to the present invention, there can also be provided a washingmethod of PASs, by which the quality of the PASs is not deteriorated.According to the present invention, there can further be provided apurification process of an organic solvent for washing. According to thepresent invention, the organic solvent for washing can be recovered,purified and used repeatedly, so that cost can be reduced withoutdeteriorating the quality of the PASs. The processes according to thepresent invention are suitable for mass-producing poly(arylene sulfides)by a great number of batches on an industrial scale and washing them.

1. A process for producing a poly(arylene sulfide), which comprises therespective steps of: (1) a polymerization step of heating andpolymerizing at least one sulfur source selected from the groupconsisting of alkali metal sulfides and alkali metal hydrosulfides and adihalo-aromatic compound in an organic amide solvent (A) after an alkalimetal hydroxide is added as needed, (2) a separation step of separatinga polymer formed from the reaction mixture containing the polymer afterthe polymerization step, (3) a washing step of washing the polymerseparated with an organic solvent (B), (4) a purification step ofpurifying the organic solvent (B) recovered after the washing step tolower the content of an alkaline compound mixed therein, and (5) a stepof recycling the organic solvent (B) purified through the washing stepof the poly(arylene sulfide).
 2. The production process according toclaim 1, wherein in the washing step, the polymer is brought intocontact with the organic solvent (B) to wash the polymer, and thepolymer is then separated from a liquid component (C) containing theorganic solvent (B) used in the washing.
 3. The production processaccording to claim 1, wherein in the separation step, after the reactionmixture containing the polymer formed is sifted to separate the polymer,a component passed through a screen and containing the organic amidesolvent (A) is separated into a solid component and a liquid component(D1), and an organic solvent (B) is then added to the solid componentseparated to separate the mixture into a liquid component (D2)containing the organic amide solvent (A) and organic solvent (B) and asolid component.
 4. The production process according to claim 2, whereinafter the washing step, the organic solvent (B) is recovered from theliquid component (C) or a mixture of the liquid component (C) and theliquid components (D1 and D2) by distillation.
 5. The production processaccording to claim 1, wherein in the purification step, an inorganicacid is added to the organic solvent (B) recovered, and the organicsolvent thus treated is distilled, thereby lowering the content of thealkaline compound.
 6. The production process according to claim 5,wherein the inorganic acid is hydrochloric acid.
 7. The productionprocess according to claim 5, wherein the inorganic acid is added to theorganic solvent (B) recovered to adjust the pH of the organic solvent tolower than 10.0, and the organic solvent thus adjusted is thendistilled.
 8. The production process according to claim 1, wherein inthe purification step, the organic solvent (B) recovered is brought intocontact with activated carbon, thereby lowering the content of thealkaline compound.
 9. The production process according to claim 1,wherein in the purification step, the content of the alkaline compoundin the organic solvent (B) recovered is lowered to at most 3,000 ppm byweight.
 10. The production process according to claim 1, wherein theorganic solvent (B) used in the washing step is acetone.
 11. Theproduction process according to claim 1, wherein the alkaline compoundis methylamine.
 12. The production process according to claim 11,wherein in the purification step, the content of methylamine in theorganic solvent (B) recovered is lowered to at most 3,000 ppm by weight.13. The production process according to claim 1, wherein a dehydrationstep of heating and dehydrating a mixture containing the organic amidesolvent, at least one sulfur source selected from the group consistingof alkali metal sulfides and alkali metal hydrosulfides and the alkalimetal hydroxide added as needed to control a water content in themixture is arranged as a step prior to the polymerization step.
 14. Theproduction process according to claim 1, wherein in the polymerizationstep, the polymerization reaction is conducted by an at least two-stagepolymerization process comprising: (I) Step 1 of heating a reactionmixture containing the organic amide solvent, the sulfur source and thedihalo-aromatic compound to 170 to 270° C. in the presence of water inan amount of 0.5 to 2.0 mol per mol of the sulfur source charged toconduct a polymerization reaction, thereby forming a prepolymer that aconversion of the dihalo-aromatic compound is 50 to 98%, and (II) Step 2of controlling the amount of water in the reaction system so as to bringabout a state that water exists in a proportion of more than 2.0 mol,but up to 10 mol per mol of the sulfur source charged, and heating thereaction system to 245 to 290° C., thereby continuing the polymerizationreaction.
 15. A process for producing a poly(arylene sulfide),comprising the step of washing the poly(arylene sulfide) obtained by apolymerization step in an organic amide solvent (A) with an organicsolvent (B), wherein the washing is conducted with an organic solvent(B), the content of an alkaline compound in which has been lowered to atmost 3,000 ppm by weight, to provide a poly(arylene sulfide) having ayellow index (YI) of at most 15.0.
 16. The production process accordingto claim 15, wherein a treatment for raising a crystallizationtemperature is conducted after the washing with the organic solvent (B)to provide a poly(arylene sulfide) having a crystallization temperature(Tmc) of at least 200° C. as measured under temperature-loweringconditions and a yellow index (YI) of at most 11.0.
 17. The productionprocess according to claim 15, wherein the organic solvent (B) used inthe washing step is recovered and recycled through the washing step ofthe poly(arylene sulfide), and at that time, an organic solvent (B), thecontent of an alkaline compound mixed in the previous washing step inwhich has been lowered to at most 3,000, is recycled through the washingstep.
 18. A method for washing a poly(arylene sulfide) obtained by apolymerization step in an organic amide solvent (A) with an organicsolvent (B), comprising recovering the organic solvent (B) used in awashing step to recycle it through the washing step, wherein the contentof an alkaline compound in the organic solvent (B) recovered at thattime is lowered to at most 3,000 ppm by weight to recycle the organicsolvent (B) thus treated through the washing step.
 19. A process forpurifying an organic solvent used in washing, which comprises washing apoly(arylene sulfide) obtained by a polymerization step in an organicamide solvent (A) with an organic solvent (B), recovering the organicsolvent (B) and then adding an inorganic acid to the organic solvent (B)recovered to conduct distillation.